Up until now rolling bearing units for sensing rotational speed are used to support a vehicle wheel freely rotatably and also to detect the rotational speed of the wheel. An example of the rolling bearing units is shown in FIG. 4.
In FIG. 4, an inner ring means comprises a hub 1. The hub 1 is provided with a flange 2 radially extending from an outer peripheral surface thereof for supporting a wheel and a disc rotor of a braking device (not shown). A first inner raceway 3a is formed directly on an axially central portion of the outer peripheral surface of the hub 1, while a second inner raceway 3b is formed on an outer peripheral surface of an inner ring 4 which is fittingly engaged on an axially inner end portion (right end portion in FIG. 4) of the hub 1. A shoulder portion 6 is formed on an outer peripheral face of the hub 1 between the first and second raceways 3a and 3b.
A nut 5 is threaded onto a male-threaded portion formed on an outer peripheral surface of the axially inner end portion of the hub 1, and the inner ring 4 is fixedly clamped between the nut 5 and the shoulder portion 6.
A cylindrical portion 7 with a cylindrical outer peripheral face is formed on the axially inner end of the hub 1 so as to protrude axially inward through the nut 5 (toward the right in FIG. 4).
A disk-shaped or annular pulse rotor 8 made of a magnetic material such as steel is press fitted onto the protruding cylindrical portion 7 of the hub 1. A ribbed portion 9 having alternate grooves and ridges in a circumferential direction is formed on the axially inner face (right side face in FIG. 4) of the pulse rotor 8.
An outer ring means comprises a cylindrical outer ring 10 which is provided around the hub 1 with a space 14 therebetween. The outer ring 10 is provided with a flange 11 radially extending from an outer peripheral face thereof and supported by a suspension unit arm (not shown) by means of the flange 11.
First and second outer raceways 12a and 12b are formed on respective portions of the inner peripheral surface of the outer ring 10 to oppose the inner ring raceways 3a and 3b, respectively. A plurality of rolling bodies 13 are provided in the space 14 between the inner raceways 3a and 3b and the outer raceways 12a and 12b, so that the hub 1 is free to rotate inside the outer ring 10.
Furthermore, a seal 27 is supported in the opening section of the axially outer end (left end in FIG. 4) of the outer ring 10. The seal 27 has an inner rim which comes into slidable contact with a portion of the outer peripheral surface of the hub 1, thereby covering the outer end opening of the space 14 to accommodate the plurality of rolling bodies 13.
Further, the axially inner end portion (right end in FIG. 4) of the outer ring 10 is provided with a metal cover 15 formed through deep drawing and engagingly fitted into the opening of the end portion of the outer ring 10.
A rotational speed sensing sensor 16 is supported on the metal cover 15 such that it faces the ribbed portion 9 on the pulse rotor 8.
It should be noted that the hub 1 rotates relative to the outer ring 10 while the cover 15 remains stationary with the outer ring 10 and that the sensor 16 is adapted to detect the rotational speed of the hub 1.
The sensor 16 comprises a pole piece 17 of magnetic material. The pole piece 17 is provided with an axially outer end face (left end face in FIG. 4) which faces the ribbed portion 9 of the pulse rotor 8. The sensor 16 also has a coil 18 which is wound around the periphery of the pole piece 17, and a permanent magnet 19 the axially outer end face of which abuts against the axially inner end face (right end face in FIG. 4) of the pole piece 17. The permanent magnet 19 is magnetized in the axial direction (left to right in FIG. 4), so that the pole piece 17 is arranged in contact with one pole of the permenent magnet 19.
With the rolling bearing unit for sensing rotational speed constructed as above, rotation of the hub 1 inside the outer ring 10 is made possible by means of the plurality of rolling bodies 13 provided between the inner raceway 3a and 3b and the outer ring raceway 12a and 12b. Furthermore, the rotational speed of the hub 1 is sensed by the sensor 16 as detailed hereinafter.
That is to say, a voltage is induced in the coil 18 wound around the pole piece 17 of the sensor 16, changing in accordance with the alternative change in distance between the axially outer end face of the pole piece 17 and the ribbed portion 9 on the axially inner end face of the pulse rotor 8. The frequency of the alternative change of the voltage is proportional to the rotational speed of the hub 1 to which the pulse rotor 8 is mounted. Hence if the signal from both ends of the coil 18 is input to a controller (not shown in the figure), the rotational speed of a wheel fixed to the hub 1 may be obtained and used for control of e,g, an antilock braking system (ABS) or traction control system (TCS).
With the conventional rolling bearing unit for sensing rotational speed constructed as above, however, the output from the rotational speed sensor 16 is generally not sufficient even when the wheel and therefore the hub 1 are rotating.
Specifically, in order to obtain a high output, it is necessary to have a high magnetic flux density passing through the pole piece 17 which is wound by the coil 18. With the conventional construction, the magnetic flux emits from the inner end face of the permanent magnet 19 (right end of FIG. 4) as shown in FIG. 5, and passes through the composite resin 20 (FIG. 4) in which the sensor 16 is buried and through the air space. It then reaches the pulse rotor 8 and returns to the outer end face of the permanent magnet 19 via the pole piece 17 as shown in FIG. 5.
With the conventional construction, the magnetic flux from the permanent magnet 19 must pass through the composite resin 20 having a high magnetic reluctance and the air space, covering a relatively long distance, so that over this distance, the magnetic flux is weakened. This results in a reduction in the flux density in the pole piece 17. Consequently even when the pulse rotor 8 rotates, a sufficiently high voltage is not produced, resulting in that wheel speed detection becomes unreliable.